Computer display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a computer display screen or portion thereof with a transitional graphical user interface showing a first image in a first sequence according to the claimed design; and,

FIG. 2 is a front view showing a second image thereof.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-2. The process or period in which an image transitions to another image forms no part of the claimed design.

The longer broken line showing of a computer and the intermediate-length broken lines showing a perimeter of a display screen of the computer and portions of the graphical user interface are included for the purpose of illustrating environmental structure and form no part of the claimed design. 

CLAIM The ornamental design for a computer display screen or portion thereof with a transitional graphical user interface, as shown and described. 